Various types of control circuits have been developed for energizing an electromagnet for a lifting operation and then automatically applying a reverse current to the magnet after the residual magnetism or potential has been removed and thereafter discontinuing the application of the reverse current. One such control circuit is illustrated in Wertz U.S. Pat. No. 2,181,539. The circuit disclosed therein includes a non-linear resistor known in the industry under the trade name of "Thyrite" which has the property of decreasing its ohmic resistance as the potential across the resistor decreases. This type of control circuit has the advantage of reducing the time required for removing the residual magnetism or potential at the end of a lifting operation and produce reverse current flow to release the material being transported.
Another type of control circuit for an electromagnet is disclosed in Griffes U.S. Pat. No. 3,154,723 wherein a non-linear circuit or resistor is again utilized for reducing the residual potential in the magnet at the end of a lifting operation and also includes an automatic reversal of current flow when the residual potential in the electromagnet drops below a predetermined level.
One of the areas that has received a large amount of attention in electromagnet controllers is to reduce the time involved for reversing the current flow within the magnet. It will be appreciated that when an electromagnet is associated with a crane, which is the normal lifting unit, the crane is being utilized constantly for transporting the material from one site to another. It is also important that all of the material be cleanly removed or dropped at the end of a lifting cycle to reduce the time and attention of the operator.